$(url_refs) - References to URLs

URLs of defined <out> sections and <media> items can be inserted
into the current content using this reference format.

$(name)

Note that all URL references are written relatively; so a file created in the
foo/bar/baz subdirectory which contains a URL reference to
blah/argh.html will be rewritten to refer to ../../../blah/argh.html.

Again, if you're not sure a URL exists, a default value can be supplied,
using this format: